This invention relates to a catalytic reactor wherein a liquid phase reactant is contacted with a gaseous phase reactant. In particular, it relates to an improvement in equipment for contacting multi-phase reactants in a fixed porous catalyst bed under continuous operating conditions, including apparatus for controlling liquid fluid flow in the reactor.
Chemical reactions between liquid and gaseous reactants present difficulties in obtaining intimate contact between phases. Such reactions are further complicated when the desired reaction is catalytic and requires contact of both fluid phases with a solid catalyst. In the operation of conventional concurrent multiphase reactors, the gas and liquid under certain circumstances tend to travel different flow paths. The gas phase can flow in the direction of least pressure resistance; whereas the liquid phase flows by gravity in a trickle path over and around the catalyst particles. Under conditions of low liquid to gas ratios, parallel channel flow and gas frictional drag can make the liquid flow non-uniformly, thus leaving portions of the catalyst bed underutilized due to lack of adequate wetting. Under these circumstances, commercial reactor performance can be much poorer than expected from laboratory studies in which flow conditions in small pilot units can be more uniform.
The segregration of the liquid and gaseous phases in a non-uniform manner in a commercial reactor is sometimes referred to as maldistribution. Attempts have been made to avoid maldistribution, such as the provision of multiple layers of catalyst with interlayered redistributors located along the reactor longitudinal axis. Numerous multi-phase reactor systems have been developed wherein a fixed porous be of solid catalyst is retained in a reactor. Typically, fixed bed reactors have been arranged with the diverse phases being passed cocurrently over the catalyst, for instance as shown in U.S. Pat. Nos. 4,126,539 (Derr et al), 4,235,847 (Scott), 4,283,271 (Garwood et al), and 4,396,538 (Chen et al). While prior reactor systems are satisfactory for certain needs, efficient multi-phase contact has been difficult to achieve for some fixed bed applications when maldistribution occurs as the reactants progress through the catalyst bed, particularly in those instances when the liquid phase is small compared to the gaseous phase. This phenomena of maldistribution developing as reactants pass through the catalyst bed can occur in commercial size large diameter reactors but is not seen in small diameter laboratory units.
In the petroleum refining industry, multi-phase catalytic reactor systems have been employed for dewaxing, hydrogenation, desulfurizing, hydrocracking, isomerization and other treatments of liquid feedstocks, especially heavy distillates, lubricants, heavy oil fractions, residuum, etc. In the following description, emphasis is placed on a selective hydrodewaxing process, which employs a catalyst comprising a medium pore siliceous zeolite having a constraint index of about 1 to 12, for example, an acidic ZSM-5 type pentasil aluminosilicate having a silica to alumina mole ratio greater than 12. It is an object of the present invention to provide a unique reactor system, including concurrent operating techniques and apparatus, adapted for treatment of liquid with a gaseous reactant in a reactor containing a porous fixed bed of solid catalyst. It is a further object to provide means for operatively connecting portions of multi-phase reactors under controlled flow conditions to maintain substantially uniform gas-liquid contact, while minimizing flow and temperature maldistribution patterns and providing downwardly gravitating liquid under substantially uniform and sufficient liquid flux to assure relatively uniform wetting of the catalyst particles. When uniform wetting of the catalyst particles are achieved, commercial reactor performance will be improved to minimize adverse reactions such as coke formation or non-selective cracking reactions and thereby match pilot unit performance.